Oxidation catalyst and process for its preparation

ABSTRACT

An oxidation catalyst formed from V 2  O 5  and TiO 2  of rutile structure is obtained by a process which comprises the following stages: 
     (a) preparation of a solution of Ti(IV) by dissolution (partial hydrolysis) of TiCl 4  in an aqueous solution at a final pH below 1.0, 
     (b) preparation of a solution of V(IV) by heating, in order to dissolve it, solid V 2  O 5  suspended in an aqueous oxalic acid solution, 
     (c) precipitation of metatitanic acid from the Ti(IV) solution at a pH not higher than 1.3, 
     (d) precipitation of vanadium oxide from the said V(IV) solution in the presence of the metatitanic acid obtained in (c), and 
     (e) separation of the precipitate, drying and calcination thereof at an elevated temperature. 
     The process provides a catalyst formed from V 2  O 5  and TiO 2  of rutile structure, which has a specific surface area of the order of 10 to 60 m 2  /g, is highly active, selective and stable under conditions of catalytic oxidation, especially under the conditions of the oxidation of o-xylene to phthalic anhydride.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

The present invention relates to an oxidation catalyst formed from V₂ O₅(vanadic anhydride or vanadium pentoxide) and TiO₂ (titanium dioxide) ofrutile structure, to the process for the preparation of such a catalystand to the use of the same catalyst in catalytic oxidation processes,especially in the process for oxidizing o-xylene to phthalic anhydride.

2. Background Art

Oxidation catalysts consisting of vanadium and titanium oxides, whichmay contain an oxidation promoter, especially an alkali metal halide,are known from the state of the art.

These known catalysts can be obtained by impregnating a preformed TiO₂support of rutile structure with an aqueous solution of a decomposablevanadium salt and decomposing the vanadium compound at an elevatedtemperature to obtain the corresponding oxide. These catalysts show ingeneral a low activity in oxidation processes, presumably due to the lowsurface area of the support. In fact, the industrial preparation ofrutile takes place by calcination at very high temperatures, of theorder of 900° to 1,000° C., of anatase TiO₂ obtained by the sulfate orchloride process, and such a calcination at elevated temperatures leadsto a diminution of the surface area of the rutile.

In practice, the assumption that the rutile structure of the TiO₂ leadsto poorer characteristics for the catalyst derives essentially from theexperimental evidence that the transformation of the anatase structureinto the rutile structure in the catalysts formed from vanadium andtitanium oxides leads to the following undesirable phenomena: a sharpdecrease in the values of the surface area, destruction of the porousstructure (macropores) and formation of a solid solution of V⁴⁺ in therutile lattice, with consequent destruction of the so-called "monolayer"of vanadium on the anatase.

These phenomena contribute to the loss of activity of the catalyst inthe oxidation of o-oxylene to phthalic anhydride, and it has beenreported that the formation of certain percentages of rutile in thecatalyst lead to a significant reduction in the useful product yieldfrom the reaction.

It is therefore generally preferred to use catalysts formed from oxidesof vanadium and titanium containing TiO₂ of anatase structure, and thesecatalysts can be obtained either by impregnation of preformed anatasewith an aqueous solution of a vanadium salt or by coprecipitation of thehydrated titanium and vanadium oxides from a solution of thecorresponding soluble salts, followed by drying of the precipitate thusobtained and calcination of the dried precipitate at elevatedtemperature.

In the technical and patent literature, the fact has been widelystressed that the catalysts for the oxidation of o-xylene to phthalicanhydride, based on V₂ O₅ and TiO₂, require the anatase structure forthe TiO₂ namely in order to obtain active and selective catalysts, andreference is made in this respect to the descriptions by: I. E. Wachs etal., Applied Catalysis, 15 (1985) 339; J. Haber, Pure and AppliedChemistry, 56 (12) (1984) 1663; M. S. Wainwright et al., Catal.Rev.-Sci. Eng., 19 (1977)(211; and also the descriptions in U.S. Pat.Nos. 4,228,038 and 4,397,768 and German Patent No. 1,553,728.

Various theories have been advanced to explain the need to have anataseTiO₂ in the catalysts under discussion, such as, for example, thepresence of a crystallographic similarity between the most exposedsurface planes in TiO₂ and the crystallographic planes of the V₂ O₅which contain the active and selective sites. Reference is made in thisrespect to the description by A. Vejux et al., J. Solid State Chem., 23(1978) 93.

On the other hand, it must be remembered that the presence of vanadiumin the catalyst favors the transformation of anatase TiO₂ to rutile andlowers the transformation temperature from 900° to 1,000° C. down to500°-600° C.

Therefore, in spite of accurate control of the temperature profile andthe hot spots in the catalyst bed, when the catalysts based on vanadiumand titanium oxides are used, a partial transformation of the anataseinto rutile, with a consequent reduction in the performance of thecatalyst, takes place in the course of a certain period of time.

BROAD DESCRIPTION OF THE INVENTION

It is the main object of the invention to provide a V₂ O₅ and TiO₂catalyst which, while containing TiO₂ in the rutile structure, overcomesthe disadvantages of the analogous catalysts of the state of the art.Another object of the invention is to provide a V₂ O₅ and TiO₂ catalystwhich is suitable for operation at temperatures lower than thosenormally used in industrial oxidation processes, in particular in theoxidation of o-xylene to phthalic anhydride, while maintaining the highproductivity and conversions at the industrially desirable level. Afurther object of the invention is therefore, to provide a catalyst ofthe above-mentioned type which has a longer service life. Yet anotherobject of the invention is to provide a process for preparing a V₂ O₅and TiO₂ catalyst which allows the TiO₂ to be obtained directly in therutile structure and leads therefore to a catalyst having theabove-mentioned advantages.

These and further objects, which become more apparent in the followingtext, are achieved by an oxidation catalyst, which comprises vandiumpentoxide and titanium dioxide of rutile structure, the said catalystcontaining 1 to 50 percent by weight of vanadium pentoxide and having asurface area in the range from 10 to 60 m² /g.

According to a further subject of the invention, the above-mentionedobjects are achieved by a process for preparing an oxidation catalyst(which is described generally and in detail below) and which comprisesthe following stages:

(a) preparation of a solution of Ti(IV) by partial hydrolysis of TiCl₄in an aqueous solution at a temperature below 50° C. and a final pHbelow 1.0;

(b) preparation of a solution of V(IV) by dissolving, by means ofheating, solid V₂ O₅ suspended in an aqueous oxalic acid solution;

(c) precipitation of metatitanic acid from the said Ti(IV) solution,while maintaining the pH at no higher than 1.3;

(d) precipitation of vanadium oxide from the said V(IV) solution in thepresence of the metatitanic acid obtained in (c); and

(e) separation of the precipitate obtained in (d), drying andcalcination thereof at a temperature above 300° C.

According to an embodiment of the process, which is the subject of theinvention, the procedure starts with the precipitation of metatitanicacid [stage (c)] from a mixture of the Ti(IV) and V(IV) solutionsprepared in (a) and (b) respectively. In this case, owing to theconditions used, which are explained in more detail below, a selectiveprecipitation of metatitanic acid takes place in the absence orsubstantial absence of precipitation of the vanadium.

According to an alternative embodiment of the process of the invention,the metatitanic acid is precipitated from the Ti(IV) solution in theabsence of the vanadium compound, the V(IV) solution is then added tothe suspension thus obtained and, finally, the vanadium oxide isprecipitated onto the metatitanic acid.

The oxidation catalyst of the invention comprises vanadium pentoxide andtitanium dioxide of rutile structure, the said catalyst containing 1 to50 percent by weight of vanadium pentoxide and having a surface area inthe range from 10 to 60 ² /g. Suitably the catalyst contains 10 to 20percent by weight of vanadium pentoxide. Suitable the catalyst alsocontains at least one oxidation promoter selected from the groupcomprising potassium, rubidium, cesium, antimony, boron, phosphorus andmixtures thereof.

The process and the catalyst according to the invention are explained inmore detail in the text which follows.

Stage (a)

According to the process of the invention, a Ti(IV) solution is preparedby dissolution (partial hydolysis) of TiCl₄ in an aqueous hydrochloricacid solution. The final pH of the solution must be below 1.0 and ispreferably about 0. The temperature during the dissolution must bemaintained at values below about 50° C. and preferably within the rangefrom 20° to 40° C.

If the temperature and pH conditions are not adhered to, totalhydrolysis of TiCl₄ with precipitation of metatitanic acid can takeplace.

The hydrolysis of TiCl₄ can also be carried out in water, under thetemperature condition indicated above, in the absence of addedhydrochloric acid. In fact, liberation of hydrochloric acid takes placeduring the partial hydrolysis of TiCl₄, and this takes place during thepartial hydrolysis of TiCl₄, and this takes the acidity to values atwhich redissolution of the titanic acid, which may be precipitated, canoccur. Nevertheless, in this case, the Ti(IV) solution obtained is lessstable in time and leads in the course of a few days to theprecipitation of TiO(OH)₂. Instead, when operating according to thepreferred embodiment in the presence of HCl, a Ti(IV) solution which isstable for several months is obtained.

Stage (b)

According to the process of the invention, a V(IV) solution is preparedby heating, in order to dissolve it, solid V₂ O₅ suspended in an aqueoussolution of oxalic acid. The initial molar ration of oxalic acid andvanadium is advantageously maintained at a value in the range from 0.5/1to 2/1, preferably about 1.5/1. In fact, an oxalic acid quantity lowerthan the above-mentioned ratios leads to an incomplete reduction of thevanadium and, in such a case, a step for separating off the unreducedexcess V₂ O₅ is necessary. A great quantity of oxalic acid can have anadverse effect in the subsequent precipitation of the metal oxides andlead to a final catalyst in which the TiO₂ contains a certain fractionof undesired anatase structure.

In general, the V₂ O₅ suspension in the aqueous oxalic acid solution isheated to a temperature between 40° and 70° C. and, under theseconditions, a complete or substantially complete reduction of V₂ O₅ toV(IV) (blue solution) is achieved in a period of time from 20 minutes toabout 2 hours, depending on the temperature applied.

Stage (c)

According to the process of the invention, metatitanic acid Tio(OH)₂ isprecipitated from the solution obtained in stage (a).

According to a possible embodiment of the process, the precipitation ofmetatitanic acid is carried out before mixing of the aqueous solutionsobtained in stages (a) and (b) in proportions depending on the desiredvalue of the Ti/V ratio in the resulting solution, which varies from 1to 100, and hence in the catalyst obtained at the end. The quantity ofvanadium in the catalyst, expressed as a percentage by weight of V₂ O₅,can in general vary from 1 percent to 50 percent and is preferably ofthe order of 10 to 20 percent.

Conveniently, the mixing of the solutions is carried out at ambienttemperature values (20° to 25° C.) and the pH value of the resultingsolution is below 1.0 and preferably is about 0.

The metatitanic acid is precipitated by addition of an alkalizing agentwhich takes the pH from the initial value (which is below 1.0 andpreferably about zero) up to a value no higher than 1.3. The preferredalkalizing agent is ammonia, the use of which makes it possible to avoidthe introduction of metallic cations into the solution. Moreover, theaddition is carried out while maintaining the mixture under vigorousstirring, and preferably the alkalizing agent is added gradually inorder to allow better pH control. In this treatment, an initialopalescense appears which increases progressively and finally leads tothe complete or substantially complete precipitation of metatitanicacid. The precipitation temperature can vary from ambient values (20° to25° C.) to the boiling point of the solution. Nevertheless, ambient orslightly higher temperature values are preferred. The time required forachieving complete precipitation of metatitanic acid depends essentiallyon the preselected temperature of the precipitation and on the final pHvalue, and it is generally in the range from 1 to 24 hours.

It is to be noted that the pH value during the phase of precipitatingthe metatitanic acid is critical. In fact, if the pH value of about 1.3is exceeded, orthotitanic acid (Ti(OH)₄) is precipitated which, in thesubsequent calcination treatment of the catalyst, leads to the formationof TiO₂ of anatase structure.

According to an alternative form of stage (c), the metatitanic acid isprecipitated by thermal hydrolysis of the solution resulting from themixing of the Ti(IV) and V(IV) solutions, operating at pH values below1.0 and preferably around zero. In this case, the mixture of the twosolutions is heated to a temperature from about 55° C. up to atemperature just below the boiling point of the mixture (about 100° C.),values of the order of 80° to 85° C. being preferred. Under theseconditions, a substantially complete precipitation of metatitanic acidis achieved in a time of the order of 1 hour.

Operating under the conditions described above leads to a selectiveprecipitation of Ti in the absence of precipitation of vanadium or atleast substantial precipitation thereof, since only small traces ofvanadium can remain combined with the metatitanic acid precipitate.

According to a further possible embodiment of stage (c), the metatitanicacid is precipitated from the Ti(IV) solution in the absence of thevanadium compound but following in other respects the procedure reportedabove. In this case, a suspension of the precipitate in the aqueous"mother" solution is obtained, to which the V(IV) solution is thenadded, and the mixture obtained is treated in the next stage (d) asexplained below.

Stage (d)

In this stage, the vanadium oxide is precipitated in its hydrated formon the metatitanic acid either from the solution, from which themetatitanic acid was previously precipitated, or from the mixture ofmetatitanic acid precipitated beforehand with the V(IV) solution.

For this purpose, an alkalizing agent, for example ammonia, is added tothe suspension resulting from the preceding stage, in order to take thepH to a value of about 5 to cause the vanadium to precipitate in theform of hydrated oxide. Preferably, the alkalizing agent is addedgradually to the suspension of metatitanic acid, with continuedstirring, at a temperature from ambient (20° to 25° C.) to about 90° C.

Alternatively, the precipitation of the vanadium oxide is carried out byevaporating the solvent under atmospheric pressure or reduced pressure.

Stage (e)

The precipitate obtained in stage (d) is separated off, dried andcalcined.

The drying is conveniently carried out at temperatures below about 150°C. and can be preceded by a washing of the precipitate with water.

The calcination is carried out at a temperature above 300° C. andpreferably at a temperature from 380° to 500° C., for a time longer than1 hour and preferably of the order of 5 hours.

This gives the catalyst according to the invention, formed from V₂ O₅and TiO₂, with a percentage of V₂ 0₅ of 1 percent to 50 percent byweight and preferably of the order of 10 to 20 percent, and containingTiO₂ of rutile structure and having a surface area of the order of 10 to60 m² /g.

The catalyst according to the invention can also contain an oxidationpromoter in addition to the V₂ O₅ and rutile TiO₂. Such a promoter,which can be selected from those known from the state of the art, suchas, for example, phosphorus, rubidium, cesium, potassium, boron,antimony, molybdenum, tungsten and their precursors or mixtures thereof,is added in stage (a) or (b) or in that of mixing the Ti(IV) and V(IV)solutions, in such quantities that it is present in the finishedcatalyst in the percentages known from the state of the art, for examplewithin the range from 0.1 to 10 percent.

The catalyst of the invention exhibits an exceptionally high stabilityunder oxidation conditions, is suitable for operation at temperatureslower than those normally used on the industrial scale, thus leading tohigh productivity and conversions, and in addition has an extendedservice life in the production cycle.

The said catalyst is particularly useful in the processes for theoxidation of o-xylene to phthalic anhydride.

DETAILED DESCRIPTION OF THE INVENTION

The experimental examples which follow are illustrative and do notrestrict the scope of the invention.

EXAMPLE 1

10 g of V₂ O₅ (vanadic anhydride) are suspended in a solution of H₂ C₂O₄.2H₂ O (20.8 g in 200 cm³ of distilled H₂ O) in a three-necked glassflask; the suspension is heated, with stirring and under a nitrogenstream, to 70° C. by means of a heating jacket, until the V₂ O₅ has beencompletely reduced to V(IV), that is to say until a clear blue solutionis obtained. The time needed for complete reduction is about 2 hours.The solution is cooled and any unreacted residues of V₂ O₅ are separatedoff by filtration. 50 cm³ of TiCl₄ are slowly added dropwise to a secondflask immersed in a water/ice bath and containing a solution of HCl inH₂ O (10 cm³ of 37 percent HCl in 300 cm³ of H₂ O); the solution is keptunder vigorous stirring during the partial hydrolysis of the TiCl₄.

The two solutions are then mixed while cold.

The precipitation is carried out by adding dropwise, with stirring ofthe solution, a solution of NH₄ OH (30 percent of NH₃) until pH 1.0 isobtained in the solution. An initial opalescense forms, and theprecipitation of metatitanic acid is complete after about 12 hours. Bycontrast, the vanadium remains in solution and is precipitated byconcentrating, the solvent being evaporated in vacuo at 60° C.

The solid obtained is dried at 80° C. for 24 hours and calcined at 400°C. for 3 hours. The surface area of the catalyst obtained in the end is45 m² /g. The TiO₂ obtained is in the rutile crystal form, and thefraction of vanadium in the catalyst is 21.9 percent, expressed aspercent by weight of V₂ O₅.

EXAMPLE 2

The V(IV) and Ti(IV) solutions are prepared in a manner analogous tothat described in Example 1. By contrast, the precipitation is carriedout by heating the mixture of the two solutions to 85° C., withstirring. The hydrolysis with precipitation of metatitanic acid iscomplete after about 3 hours. The vanadium which, however, remains insolution is precipitated in the same solution by concentrating, thesolvent being evaporated in vacuo at 60° C. The precipitate is dried at80° C. for 24 hours and calcined at 400° C. for 3 hours.

EXAMPLES 3-5

The catalyst is prepared as in Example 2, but the solid product afterdrying is calcined at 450° C. for 3 hours, or at 500° C. for 3 hours orat 550° C. for 3 hours. The surface areas obtained are 30 m² /g, 20 m²/g, 14 m² /g in these three cases, respectively.

EXAMPLE 6

The catalyst is prepared analogously to Example 2, except that theinitial quantity of V₂ O₅ used for preparing the V(IV) solution is 5.0g. The final vanadium content after calcination at 400° C. for 3 hoursis 12.3 percent by weight, expressed as V₂ O₅. The surface area is 45 m²/g, and the TiO₂ is in the rutile crystal form.

EXAMPLE 7

The V(IV) and Ti(IV) solutions are prepared as indicated in Example 1;NH₄ OH (30 percent of NH₃) is then added dropwise up to pH 1.0 to theTi(IV) solution, with stirring.

An initial opalescence forms, and the precipitation of metatitanic acidis complete after about 12 hours. The V(IV) solution is then added withstirring to the suspension; vanadium oxide then precipitates in itshydrated form when the solvent is evaporated in vacuo at 60° C.

The treatment of the precipitate is analogous to that indicated inExample 1. After calcination, the catalyst shows the rutile structure ofTiO₂, has a surface area of 45 m² /g and a vanadium content of 21.9percent expressed as percent by weight of V₂ O₅.

EXAMPLE 8

The V(IV) and Ti(IV) solutions are prepared in a manner analogous tothat reported in Example 1; the Ti(IV) solution is heated to 85° C.,with stirring; the hydrolysis with precipitation of metatitanic acid iscomplete after about 2 hours. The V(IV) solution is then added withstirring to the suspension; the vanadium oxide then precipitates in itshydrated form, when the solvent is evaporated in vacuo at 60° C.

The treatment of the precipitate is analogous to that indicated inExample 1. After calcination, the catalyst shows the rutile structure ofTiO₂, has a surface area of 45 m² /g and a vanadium content of 21.9percent, expressed as percent by weight of V₂ O₅.

EXAMPLE 9

The preparation procedure is analogous to that reported in Example 7,but the metatitanic acid, once the precipitation is complete, isfiltered off and the solid residue is washed with H₂ O, dried at 80° C.for 24 hours and then calcined at 400° C. for 3 hours. This gives TiO₂in the rutile crystal form with a surface area of 40 m² /g. The solid isthen suspended in the V(VI) solution; the vanadium is precipitated inits hydrated form, when the solvent is evaporated in vacuo at 60° C. Theprecipitate is dried at 80° C. for 24 hours and calcined at 400° C. for3 hours.

EXAMPLE 10

The preparation procedure is analogous to that reported in Example 8,but the metatitanic acid, once precipitation is complete, is filteredoff and the solid residue is washed with H₂ O, dried at 80° C. for 24hours and then calcined at 400° C. for 3 hours. This gives TiO₂ in therutile crystal form with a surface area of 40 m² /g. The solid issuspended in the V(IV) solution; the vanadium is precipitated in itshydrated form, when the solvent is evaporated in vacuo at 60° C. Theprecipitate is dried at 80° C., for 24 hours and calcined at 400° C. for3 hours.

EXAMPLES 11-14

The preparation procedure is analogous to that reported in Examples 7 to10, except that the V(IV) solution is prepared by reduction of 5.0 g ofV₂ O₅. The catalysts finally obtained contain 12.3 percent by weight ofV₂ O₅.

EXAMPLES 15-17

The catalysts prepared by the procedure reported in the precedingexamples are used for the gas-phase oxidation with air of o-oxylene tophthalic anhydride.

The process is run at temperatures in the range from 290° to 330° C.,that is to say lower than the temperatures 360° to 380° C. which can beused with commercial catalysts, and at higher hourly space velocities ofthe gas of the order of about 5,000 hours⁻¹, o-xylene conversions ofabout 99 percent and higher being obtained.

These data precisely demonstrate the superiority of the catalysts whichare the subject of the present invention and which show high activitiesat temperatures about 30° 90° C. lower than those used according to thestate of the art, and at space velocities much higher than the usualones, which leads to a superior productivity and longer life of thecatalyst.

What is claimed is:
 1. An oxidation catalyst consisting of vanadiumpentoxide and titanium dioxide of rutile structure, the said catalystcontaining 1 to 50 percent by weight of vanadium pentoxide and having asurface area in the range from 10 to 60 m² /g.
 2. A catalyst as claimedin claim 1 which contains 10 to 20 percent by weight of vanadiumpentoxide.
 3. A process for preparing an oxidation catalyst, saidoxidation catalyst comprising vanadium pentoxide and titanium dioxide ofrutile structure, said catalyst containing 1 to 50 percent by weight ofvanadium pentoxide and having a surface area in the range from 10 to 60m² /g. comprising the following steps:(a) preparing a solution of Ti(IV)by partial hydrolysis of TiCl₄ in an aqueous solution at a temperaturebelow 50° C. and a final pH below 1.0; (b) preparing a solution of V(IV)by dissolving, by means of heating, solid V₂ O₅ suspended in an aqueousoxalic acid solution; (c) precipitating a metatitanic acid from saidTi(IV) solution, while maintaining the pH at no higher than 1.3; (d)precipitating of vanadium oxide from said V(IV) solution in the presenceof the metatitanic acid obtained in step (c); (e) separating of theprecipitate obtained in step (d), and drying and calcining theprecipitate at a temperature above 300° C.
 4. The process as claimed inclaim 3 wherein the partial hydrolysis step (a) is carried out in anaqueous hydrochloric acid solution to a final pH of
 0. 5. The process asclaimed in claim 3 wherein the partial hydrolysis step (a) is carriedout in water in the absence of added acid.
 6. The process as claimed inany one of claims 3 to 5 wherein the molar oxalic acid/vanadium ratiovarying from 0.5/1 to 2/1 is used in step (b).
 7. The process as claimedin any one of claims 3 to 5 wherein step (c) is carried out in theabsence of an alkalizing agent at a pH below 1 and at a temperature inthe range from 55° to 95° C.
 8. The process according to any one ofclaims 3 to 5 which comprises, before the said step (c), a stage ofmixing the said Ti(IV) and V(IV) solutions, prepared in steps (a) and(b) respectively, in such a ration that a Ti/V atomic ratio from 1 to100 is obtained in the resulting mixture, the said step (c) carried outon this mixture leading to a selective precipitation of metatitanicacid.
 9. The process as claimed in any one of claims 3 to 5 wherein thesaid step (d) is carried out by subjecting a mixture of the said V(IV)solution and the metatitanic acid precipitated in step (c) toevaporation under a pressure which varies from atmospheric pressure to areduced pressure.
 10. The process as claimed in any one of claims 3 to 5which also comprises the addition, in one of steps (a) and (b) or duringthe mixing of the Ti(IV) and V(IV) solutions before step (c), of anoxidation promoter selected from the group comprising potassium,rubidium, cesium, antimony, boron, phosphorus, molybdenum, tungsten,precursors thereof and mixtures thereof.
 11. A process for preparing anoxidation catalyst containing 1 to 50 percent by weight of V₂ O₅, theremainder being TiO₂ of rutile structure, and having a surface area of10 to 60 m² /g, which comprises the following steps:(a) preparing asolution of Ti(IV) by partial hydrolysis of TiCl₄ in an aqueous solutionat a temperature below 50° C. and a final pH below 1.0; (b) preparing asolution of V(IV) by dissolving, by means of heating, solid V₂ O₅suspended in an aqueous oxalic acid solution;(i) mixing of saidsolutions prepared in steps (a) and (b) to obtain a mixture containing aTi/V atomic ratio in the range from 1 to 100; (c) selectivelyprecipitating metatitanic acid from said mixture, while maintaining thepH at no higher than 1.3; (d) precipitating vanadium oxide on themetatitanic acid from the solution obtained in step (c)l; and (e)separating the coprecipitate obtained in step (d), drying and calciningthe precipitate at a temperature above 300°C.
 12. A process forpreparing an oxidation catalyst containing 1 to 50 percent by weight ofV₂ O₅, the remainder being TiO₂ of rutile structure, and having asurface area of 10 to 60 m² /g, which comprises the following steps:(a)preparing a solution of Ti(IV) by partial hydrolysis of TiCl₄ in anaqueous solution at a temperature below 50° C. and a final pH below 1.0;(b) preparing a solution of V(IV) by dissolving, by means of heating,solid V₂ O₅ suspended in an aqueous oxalic acid solution; (c)precipitating metatitanic acid from said Ti(IV) solution, whilemaintaining the pH at no higher than 1.3;(ii) mixing of the productobtained in step (c) with the V(IV) solution obtained in step (b); (d)precipitating vanadium oxide on the metatitanic acid from the mixtureobtained in (ii); and (e) separating the coprecipitate obtained in (d),drying and calcining the precipitate at a temperature above 300° C. 13.An oxidation catalyst comprising vanadium pentoxide, titanium dioxide ofrutile structure, and an oxidation promoter selected from the groupconsisting of (i) potassium, (ii) rubidium, (iii) cesium, (iv) antimony,(v) boron and (vi) mixtures of oxidation promoters (i) to (v), saidcatalyst containing 1 to 50 percent by weight of said vanadium pentoxideand having a surface area in the range from 10 to 60 m² /g.
 14. Thecatalyst as claimed in claim 13 wherein said catalyst contains 10 to 20percent by weight of said vanadium pentoxide.
 15. The catalyst asclaimed in claim 14 wherein said catalyst contains 0.1 to 10 percent byweight of said oxidation promoter.
 16. A process for preparing anoxidation catalyst, said oxidation catalyst comprising vanadiumpentoxide and titanium dioxide of rutile structure, said catalystcontaining 1 to 50 percent by weight of vanadium pentoxide and having asurface area in the range from 10 to 60 m² /g, comprising the followingsteps:(a) preparing a solution of Ti(IV) by partial hydrolysis of TiCl₄in an aqueous solution at a temperature below 50° C. and a final pHbelow b 1.0; (b) preparing a solution of V(IV) by dissolving, at atemperature of 40° to 70° C., solid V₂ O₅ suspended in an aqueous oxalicacid solution; (c) precipitating a metatitanic acid from the Ti(IV)solution, while maintaining the pH at no higher than 1.3; (d)precipitating vanadium oxide from said V(VI) solution in the presence ofthe metatitanic acid obtained in step (c); (e) separating theprecipitate obtained in step and (d), drying and calcining theprecipitate at a temperature above 300° C.
 17. The process as claimed inclaim 16 wherein the partial hydrolysis step (a) is carried out in anaqueous hydrochloric acid solution to a final pH of
 0. 18. The processas claimed in claim 16 wherein the partial hydrolysis step (a) iscarried out in water in the absence of added acid.
 19. The process asclaimed in claim 18, wherein the molar oxalic acid/vanadium ratiovarying from 0.5/1 to 2/1 is used in step (b).
 20. The process asclaimed in claim 16 wherein the catalyst also contains an oxidationpromoter selected from the group consisting of (i) potassium, (ii)rubidium, (iii) cesium, (iv) antimony, (v) boron and (vi) mixtures ofoxidation promoters (i) to (v).
 21. A process for preparing an oxidationcatalyst, said oxidation catalyst comprising vanadium pentoxide andtitanium dioxide of rutile structure, said catalyst containing 1 to 50percent by weight of vanadium pentoxide and having a surface area in therange from 10 to 60 m² /g, comprising the following steps;(a) preparinga solution of Ti(IV) by partial hydrolysis of TiCl₄ in an aqueoussolution at a temperature below 50° C. and final pH below 1.0; (b)preparing a solution of V(IV) by dissolving, by means of heating, solidV₂ O₅ suspended in an aqueous oxalic acid solution; (c) precipitating ametatitanic acid from said Ti(IV) solution, while maintaining the pH atno higher than 1.3, the metatitanic acid precipitation being carried outby introducing an alkalizing agent into the Ti(IV) solution until a pHno higher than 1.3 is reached, the solution being stirred at atemperature in the range from 20° to 90° C.; (d) precipitating vanadiumoxide from said V(IV) solution in the presence of the metatitanic acidobtained in step (c); (e) separating the precipitate obtained in step(d), and drying and calcining the precipitate at a temperature above300° C.
 22. The process as claimed in claim 21 wherein said alkalizingagent is ammonia.
 23. The process as claimed in claim 21 wherein thepartial hydrolysis step (a) is carried out in an aqueous hydrochloricacid solution to a final pH of
 0. 24. The process as claimed in claim 21wherein the partial hydrolysis step (a) is carried out in water in theabsence of added acid.
 25. The process as claimed in claim 21 whereinthe molar oxalic acid/vanadium ratio varying from 0.5/1 to 2/1 is usedin step (b).
 26. The process as claimed in claim 21 wherein the catalystalso contains an oxidation promoter selected from the group consistingof (i) potassium, (ii) rubidium, (iii) cesium, (iv) antimony, (v) boronand (vi) mixtures of oxidation promoters (i) to (v).
 27. A process forpreparing an oxidation catalyst, said oxidation catalyst comprisingvanadium pentoxide and titanium dioxide of rutile structure, saidcatalyst containing 1 to 50 percent by weight of vanadium pentoxide andhaving a surface area in the range from to 60 m² /g, comprising thefollowing steps:(a) preparing a solution of Ti(IV) by partial hydrolysisof TiCl₄ in an aqueous solution at a temperature below 50° C. and afinal pH below 1.0; (b) preparing a solution of V(IV) by dissolving, bymeans of heating, solid V₂ O₅ suspended in an aqueous oxalic acidsolution; (c) precipitating a metatitanic acid from said Ti(IV)solution, while maintaining the pH at no higher than 1.3; (d)precipitating vanadium from said V(IV) solution in the presence of themetatitanic acid obtained in step (c), said step (d) being carried outby introducing an alkalizing agent into a mixture of said V(IV) solutionand the metatitanic acid precipitated in said step (c), until a pH ofabout 5 is reached, the mixture being stirred at a temperature in therange from 20° to 90° C.; (e) separating the precipitate obtained instep (d), and drying and calcining the precipitate at a temperatureabove 300° C.
 28. The process as claimed in claim 27 wherein saidalkalizing agent is ammonia.
 29. The process as claimed in claim 28wherein the partial hydrolysis step (a) is carried out in an aqueouoshydrochloric acid solution to a final pH of
 0. 30. The process asclaimed in claim 28 wherein the partial hydrolysis step (a) is carriedout in water in the absence of added acid.
 31. The process as claimed inclaim 28 wherein the molar oxalic acid/vanadium ratio varying from 0.5/1to 2/1 is used in step (b).
 32. The process as claimed in claim 27wherein the catalyst also contains an oxidation promoter selected fromthe group consisting of (i) potassium, (ii) rubidium, (iii) cesium, (iv)antimony, (v) boron and (vi) mixtures of oxidation promoters (i) to (v).33. An oxidation catalyst consisting of vanadium pentoxide, titaniumdioxide of rutile structure, and an oxidation promoter selected from thegroup consisting of (i) potassium, (ii) rubidium, (iii) cesium, (iv)antimony, (v) boron and (vi) mixtures of oxidation promoters (i) to (v),said catalyst containing 1 to 50 percent by weight of said vanadiumpentoxide and having a surface area in the range from 10 to 60 m² /g.